Adaptive Therapy With Capecitabine for Treatment of Metastatic ER Positive, HER2 Negative Breast Cancer

Inclusion Criteria: * PRE-REGISTRATION: Provide written informed consent Note: Pre-registration should occur prior to screening research blood draws being completed * PRE-REGISTRATION: Provider anticipates the patient will begin on capecitabine within 14 days or has started capecitabine within the past 42 days and has had a maximum of two cycles Note: Pre-registration should occur prior to screening research blood draws being completed REGISTRATION - INCLUSION CRITERIA * Age ≥ 18 years * Histological confirmation of estrogen-receptor positive (ER+), HER2-negative overexpression or amplification negative as per American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines, metastatic breast cancer * Measurable disease. Bone only disease allowed if associated with soft tissue component that is measurable by Response Evaluation Criteria is Solid Tumors (RECIST) 1.1 criteria * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2 * Hemoglobin ≥ 9.0 g/dL (obtained ≤ 14 days prior to registration), no transfusions allowed ≤ 14 days prior to registration * Absolute neutrophil count (ANC) ≥ 1500/mm\^3 (obtained ≤ 14 days prior to registration) * Platelet count ≥ 100,000/mm\^3 (obtained ≤ 14 days prior to registration) * Total bilirubin ≤ 1.5 x upper limit of normal (ULN) (obtained ≤ 14 days prior to registration) * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 x ULN (≤ 5 x ULN for patients with liver …